The application of this method to geophysical prospecting by measurement of the anisotropy of propagation or reflection of shear waves in rocks will be more particularly described; nevertheless, this application is no way limitative, and the method of the invention can be applied to numerous very different fields of measurement where it can be employed each time that there is present a progressive wave having transverse components: such is particularly the case of an electro-magnetic wave, or an acoustic wave propagating in a solid medium (or comparable to a solid), which permits application of the invention to radar, sonar and medical imaging techniques.
In other words, the method of the invention is able to be applied, in a general manner, each time that there is present a medium which propagates shear waves or the like waves in a non-isotropic manner, or even an isotropic medium, when a reflection produces an anisotropy.
The phenomenon of an anisotropy of propagation of a transverse wave will first be explained taking as an example the case of propagation of a shear wave in a rock, with reference to FIGS. 1 and 2.
The phenomenon of an anisotropy of propagation of a shear wave--often designated "S wave" (for "shear" wave) is typical of fissured or stratified materials, and the study of propagation of shear waves in these materials gives valuable indications of the structure of the medium traversed.
In FIG. 1, there is illustrated an incident shear wave SH/SV having for example an orientation of 45.degree. with respect to the horizontal, then having components both in the vertical direction V and in the horizontal direction H. This incident wave penetrates into a stratified, anisotropic medium M, which creates a phenomenon of discrimination of the shear waves ("shear-wave splitting") along the propagation path: the vertical and horizontal components are propagated with different polarisations, speeds and attenuations in the medium M, thus producing, from the same incident wave SV/SH, two resultant waves SV and SH, respectively polarised vertically and horizontally, having a time displacement .tau.. Thus, the wave form, which is the vectorial sum of the two components SV and SH, gradually becomes modified during its propagation across an anisotropic medium.
In the same manner, FIG. 2 illustrates the propagation of a shear wave SN/SE propagating along a substantially vertical ray vecter, and having components both in a North direction N and an East direction E. This incident wave SN/SE penetrates into an anisotropic medium M having fissuring oriented for example in the North-South direction. As in the previous case, the two components North SN and East SE of the incident wave are propagated at different speeds because of the anisotropy, gradually modifying the form of the wave and creating a time displacement .tau.. The component propagating the fastest is that which is polarised parallel to the fissure faces (that is to say in the North-South direction, in the example chosen); further, this component is only slightly attenuated. On the other hand, the East-West oriented component is propagated more slowly and experiences a strong attenuation.
Study of the anisotropy of the medium then comprises study of the following parameters:
finding the orientations respectively corresponding to the smallest and greatest speed of propagation (finding the principal directions of anisotropy), which permits evaluation of the orientation of the fissures, which will be perpendicular to the direction of minimum horizontal compression;
the time displacement .tau. between the components corresponding to the two principal directions of anisotropy;
measurement of the attenuation of the different components, particularly their differential attenuation in the two principal directions of anisotropy.
This will provide indications of the nature of the medium traversed and permit quantification of the importance of the anisotropy, furnishing also an indication of the density of fracturing or the porosity of the medium traversed.